1. Field of the Invention
The present invention relates to a telecommunication terminal device used in connection with a public switched telephone network. Particularly, the present invention relates to a telecommunication terminal device and method of using the same, the telecommunication terminal device being capable of transmitting telecommunication signals regardless of a serial connection or parallel connection to an additional telecommunication terminal device.
2. Description of the Related Art
FIG. 1 is a block diagram of a telecommunication terminal device typically used in Korea or the United States. Referring to FIG. 1, a telecommunication terminal device 20 is connected as a main terminal device to a public switched telephone network (PSTN) 10. An external telecommunication terminal device 40 (such as a telephone, a facsimile machine, or the like) is connected as a sub terminal device to the main terminal device 20. The main terminal device 20 includes a PSTN connection unit 22 having two pins 24 and 26 through which the PSTN 10 and a feeding circuit 28 of the main terminal device 20 are connected. The main terminal device 20 also includes an external terminal connection unit 30 having two pins 32 and 34 through which the sub terminal device 40 is connected in parallel to the main terminal device 20.
FIG. 2 is a block diagram of a telecommunication terminal device typically used in Germany or Italy. Referring to FIG. 2, a telecommunication terminal device 70 is connected as a main terminal device to a PSTN 60. An external telecommunication terminal device 90 is connected as a sub terminal device to the main terminal device 70. The main terminal device 70 includes a PSTN connection unit 72 having four pins 74, 76, 78, and 80. The PSTN 60 and a feeding circuit 82 of the main terminal device 70 are connected through two pins 74 and 76 among the four pins of the PSTN connection unit 72. The external terminal device 90 is serially connected to the main terminal device 70 through the other two pins 78 and 80 of the PSTN connection unit 60.
Referring again to FIG. 1, an electrical current provided from the PSTN 10 flows into the feeding circuit 28 and returns to the PSTN 10 through the two pins 24 and 26 of the PSTN connection unit 22. That is, a current loop is formed between the PSTN 10 and the feeding circuit 28 through the two pins 24 and 26 of the PSTN connection unit 22. Likewise, referring to FIG. 2, an electrical current provided from the PSTN 60 flows into the feeding circuit 82 and returns to the PSTN 60 through the two pins 74 and 76 of the PSTN connection unit 72. That is, a current loop is formed between the PSTN 60 and the feeding circuit 82 through the two pins 74 and 76 of the PSTN connection unit 72.
Such a telecommunication terminal device connected to a public switched telephone network is disclosed in U.S. Pat. No. 5,912,964.
However, when a user uses a main terminal device 20, 70 in connection with an external terminal device 40, 90, there is a difference between the telecommunication terminal device shown in FIG. 1 and that shown in FIG. 2. That is, while the external terminal device 40 is connected to the main terminal device 20 through the pins 32 and 34 of the external terminal connection unit 30 in FIG. 1, the external terminal device 90 is connected to the main terminal device 70 through the remaining pins 78 and 80 of the PSTN connection unit 72 in FIG. 2.
As described above, the interface system of the telecommunication terminal device for connecting the external terminal device with the PSTN in the countries using a serial connection system is different that in the countries adopting a parallel connection system. Therefore, it is required to manufacture terminal devices to have different interface systems on a country or regional basis. That is, for the countries such as Korea or the United States, a parallel type terminal device having the interface system shown in FIG. 1 is required. For the countries such as Germany or Italy, a serial type terminal device having the interface system shown in FIG. 2 is required. Consequently, there is a problem in that the terminal device having the interface system shown in FIG. 1 cannot be used in the countries where the interface system shown in FIG. 2 is required.